The macnearite samples come from crosscut "Tiefstollen" in a part of gallery which has served for the Gabe-Gottes and Saint-Jacques mines arsenic ore storage.
The minerals associated with macnearite are picropharmacolite, pharmacolite, guérinite and haidingerite.
The formation of this mineral is certainly due to the arsenic acid reaction with the carbonate gangue mineral. The presence of sodium must be linked to the abundance of mirabilite that lines the walls of this gallery (WEIL, 1937).